1. Field of the Invention
The present invention relates generally to amplitude control in communication systems, and more specifically, to a modulation system and method to improve the overall efficiency when controlling amplitude of RF signals having a non-constant envelope.
2. Description of the Related Art
Radio Frequency (RF) signals having a non-constant envelope (NCE) are common in several widely deployed wireless and wired or coaxial cable protocols such as 802.11/WiFi, Bluetooth EDR (Enhanced Data Rate), 3GPP-LTE (3rd Generation Partnership Protocol—Long Term Evolution), WCDMA (Wideband Code Division Multiple Access), HSDPA/HSUPA (High Speed Downlink Packet Access/High Speed Uplink Packet Access), WiMAX (Worldwide Interoperability for Microwave Access), HomePlug (various power line communications specifications that support networking over existing home electrical wiring), DOCSIS (Data Over Cable Service Interface Specification), among many others.
Modern communications standards often rely on NCE modulation schemes because they can achieve high-spectral efficiency, allowing more data throughput for a given bandwidth. As communications applications have grown more pervasive, communications standards have evolved to incorporate modulation schemes with higher bandwidth and more complexity, such as, for example, 64-QAM (Quadrature Amplitude Modulation) OFDM (Orthogonal Frequency-Division Multiplexing) in 802.11n (amendment to the IEEE 802.11-2007 wireless networking standard by the Institute of Electrical and Electronics Engineers). These modulation schemes facilitate high spectral efficiency and high maximum data transmission rates.
The conventional RF transmitter architectures and circuit techniques, however, have not achieved high efficiency when transmitting high bandwidth NCE signals. In particular, achieving high average PA (Power Amplifier) efficiency generally becomes more difficult as the Peak to Average Power Ratio (PAPR) of the transmitted signal increases, especially at high bandwidth. Low transmitter efficiency is a significant problem in portable wireless devices in which battery lifetime is especially important. Efficiency tradeoffs are often made for some of the most common transmitter topologies in use today, such as, for example, the conventional linear transmitter and the polar transmitter with supply-modulation.
An exemplary conventional linear transmitter sums orthogonal quadrature RF signals (e.g., IRF and QRF) at an input to a variable gain amplifier (VGA) having its output coupled to the input of a PA. The sum of IRF and QRF provides a summation signal which carries substantially all of the information. The summation signal, however, typically does not have sufficient power for direct transmission. The VGA and PA are provided to amplify the summation signal without introducing excessive distortion and to drive an output load, such as an antenna or the like.
The PA and VGA should maintain a high degree of linearity to avoid distorting the output signal, hence the term “linear transmitter.” The linearity requirement is at the heart of the efficiency limitation imposed by this architecture. The PA in a linear transmitter is traditionally a Class A or Class AB topology because of their ability to maintain a linear transfer characteristic. The Class A amplifier has a well known maximum efficiency of 50% for a full-scale output. The efficiency of the Class A amplifier, however, falls very quickly as the output signal amplitude drops below full-scale. Furthermore, the transfer characteristic of the Class A amplifier becomes nonlinear as the signal amplitude approaches full-scale. Thus, the linear transmitter designer seeks to ensure that the input to the PA avoids full-scale, which is exactly where the PA achieves its best efficiency.
The nonlinearity of an open loop Class A power amplifier may be roughly quantified by a 1 decibel (dB) compression point P1dB. For a typical OFDM signal in 802.11g/n, the average PA input power should roughly be 10 dB below the compression point P1dB for a traditional Class A design, or in other words, an Output Power Backoff (OBO) of 10 dB. More recently, a variety of PA linearization and pre-distortion techniques are known that can improve the average PA efficiency by allowing for a lower OBO. The techniques essentially use feedforward and/or feedback compensation to correct for the nonlinearity of the PA transfer characteristic. While higher efficiencies are possible with the linear topology, it is fundamentally poorly suited to high-efficiency transmission of signals with large PAPR. This follows from the fact that linear PA efficiency is a super-linear function of output amplitude, resulting in extremely low efficiency at low output levels.
The efficiency limitations of linear transmitters for high PAPR signals are well known and thus several alternate, higher efficiency topologies have been proposed. The polar transmitter is one well-known topology that can theoretically transmit high PAPR signals with very high efficiency. In a polar transmitter, the RF signal is represented by its polar coordinates (i.e. amplitude and phase) rather than by Cartesian coordinates, as in the linear transmitter.
Essentially, the PA takes only one of the two polar coordinates, namely phase, at its RF input. The amplitude coordinate is controlled separately, typically by modulating the supply of a Class E, Class C or other high-efficiency PA, as these nonlinear power amplifiers respond linearly (to first order) to voltage supply variation. In terms of average efficiency, the key benefit of supply modulation is that it theoretically enables nearly constant PA efficiency versus output amplitude. There are, however, several practical challenges associated with simultaneously achieving high-efficiency, high-bandwidth, and high modulation accuracy that have made this approach impractical for many applications.
First, the amplitude response of the Class E power amplifier (and others) is not perfectly linear with supply voltage and the output phase is not constant. Further, zero output amplitude is difficult to achieve via supply modulation with this topology. These non-idealities can result in high EVM (error vector magnitude) and distortion products that degrade spectral purity. The nonlinearity of the amplitude and phase responses can be compensated digitally, but circuit-level modifications and/or dynamic reduction of the amplitude envelope of the RF input signal are required in order to reach zero output amplitude. Furthermore, the additional power consumption associated with implementing accurate digital pre-distortion can outweigh the proposed efficiency benefits of the architecture.
In contrast to the linear transmitter, the PA in the polar transmitter need not reproduce amplitude variations at its RF input linearly, and thus may operate in its deeply nonlinear, higher efficiency region, regardless of amplitude. Overall efficiency, however, is determined by the product of PA efficiency and the efficiency of the amplitude modulation (AM) circuitry. Thus, the burden of high efficiency is split between PA and the AM circuitry, and the efficiency of the amplitude modulator is critical.
High overall efficiency may be achievable by using an inductor-based switching DC-DC converter, such as a buck converter, to control the PA supply. However, in practice, this is very difficult to achieve for all but the lowest bandwidth applications. A major drawback of this approach is the bandwidth limitation imposed by the buck converter, as this implies a very high switching frequency for the converter. As modulation bandwidth, and thus switching frequency, of the DC-DC converter is increased, the efficiency of the converter degrades rapidly.
In addition, the modulation bandwidth of a switching converter is dependent on its output level, with the lowest bandwidth regions at the upper and lower extremes of the output range. This bandwidth dependence results in AM loop dynamics that vary as a function of output amplitude, which presents a serious challenge for meeting EVM and spectral mask requirements, especially as bandwidth and PAPR increase.
In light of the challenges, hybrid supply control strategies have been proposed in which a combination of a low dropout (LDO) regulator and a switching converter are used to control the PA supply. If the desired signal modulation bandwidth is far in excess of the bandwidth of the switching converter, then the overall efficiency of the system is sharply degraded. Further, hybrid methods present a difficult control problem due to the use of two separate amplitude modulator circuits. These methods generally tradeoff efficiency to achieve higher bandwidth.
What is needed is a method for achieving high bandwidth amplitude modulation that maintains high efficiency and stable dynamics across a wide range of amplitude values, thereby enabling a high efficiency transmitter that addresses the needs of high bandwidth, high PAPR signal constellations which are prevalent in modern spectrally efficient communication systems.